Attom High Resolution-ICP-MS

INSTRUMENT OVERVIEW

ION LENS (TRANSFER LENS) STACK

• The Transfer Lens system is used to:

– 1. extract the ions entering the analyzer section through the orifices of the cones with very high velocity; i.e. supersonic speed;

– 2. focus the divergent ion beam onto the target (Entrance Aperture),

– 3. correct the direction of the beam to the target (Entrance Aperture)

– 4. accelerate the ions to their full speed by applying high voltage– 5. shape the ion beam into a flat shape so as to make it through

the Entrance slit.

• The ions are subsequently travelling with the desired velocity through the focus point (Entrance Slit) and start to diverge again slightly.

The proportion of ions sampled from the plasma that make it through to thedetector is actually extremely small (1 part in 106 to 108), therefore, a high ion gain detection system is required if low detection limits are to be achieved

Ion Focusing –General Assumptions

1. All ions are free particles with positive charge2. Density of ion beam is not great enough to

induce repulsion (‘space charge’ effects)3. Presence of ions does not change electrostatic

fields4. Vacuum conditions are adequate to give ions

necessary mean free path5. Ions originate with a constant kinetic energy

Ion Optics

• In an ideal world...

• All ions leaving the plasma would have the same kinetic energy regardless of m/z ratio

• This would result in uniform ion transmission through the lens stack

Ion Optics

• In the real world...

• All ions are essentially accelerated to the supersonic velocity of Ar in the supersonic jet

• This results in a range of kinetic energy that is a function of mass

ION ENERGY vs m/zM

AX

. IO

N K

E (e

V)

m/z

Ion Optics

• What does this mean?

• A single set of ion lens settings will not be appropriate for all elements

• Therefore, must COMPROMISE (unless looking at a narrow mass range)

Space charge effects

The mutual repulsion of ions of like (similar) charge limits the total number of ions that can be compressed into a

beam of given size

Space Charge Effects• Plasma

- ion flux is balanced by the electron flux- essentially neutral

• Supersonic jet- ion flux is balanced by the electron flux- essentially neutral

• Lens stack- electrons are repelled by negative potential- ion beam gains positive charge

Space Charge Effects• Remember our assumptions

- density of ion beam not great enoughto induce space charge effects

• ASSUMPTION NOT MET-further reason for non-ideal behavior and

different response across the mass range

Question: what elements are most likely to be effected by space charge effects?

Space Charge Effects

• Space charge effect is most strongly felt by lighter mass elements

• The space charge force (positive-positive repulsion) acts on all ions equally

Space Charge Effects

• Recall most of the ions present are Ar = mass 40

• Elements lighter than mass 40 are going to undergo space charge effects even if there is no other matrix element!

Question: why are the low mass elements effected by space charge effects to a greater degree?

Space Charge Effects• Heavy elements are effected less than light

elements

• “Heavy” matrices cause more problems than light matrices

• Best case scenario = analysis of uranium in water

• Worst case scenario = analysis of Li in organic-rich solution

Detectors• The purpose of an electron multiplier is to detect every

ion of the selected mass that has passed through the energy (mass) filter of a mass spectrometer.

• The basic physical process that allows an electron multiplier to operate is referred to as secondary electron emission.

• When an ion or electron strikes a surface it can cause electrons located within the outer layers of atoms to be released. The number of secondary electrons released depends on the type of incident primary particle, its energy, and characteristic of the incident surface.

• In general, there are two basic types of electron multipliers commonly used in mass spectrometric analysis: these are discrete-dynode and continuous-dynode electron multipliers.

Detectors• Discrete-dynode electron multipliers amplify the

secondary electron emission process by using an array of electrodes referred to as dynodes.

• Ions hitting the first dynode cause secondary electrons to be emitted from the surface. The optics of the dynodes focuses these secondary electrons onto the next dynode of the array, which in turn emits even more secondary electrons from its surface than the first dynode.

• Consequently, a cascade of electrons is produced between successive dynodes, with each dynode increasing the number of electrons in the cascade by a factor of 2 to 3.

• This process is allowed to continue until the cascade of electrons reaches the output electrode where the signal is extracted.

Detectors

• For a new (unused) electron multiplier, the gain is achieved with a lower applied voltage (~1800 volts).

• With time and usage, the surfaces of the dynodes slowly become covered with contaminants from the high vacuum system, which results in a decrease of their secondary electron emission capacity (and consequently drop in ‘gain’).

• Thus, the operating high voltage applied to the electron multipliers must be periodically increased in order to maintain the required multiplier gain.

Detectors

Detectorsdiscrete dynode electron multiplier

Detector System - Attom

Detector - AttoM• Placed in front of the multiplier, is a retardation filter, and

can be used to improve the abundance sensitivity, which defines the ability to record a low intensity peak adjacent to an intense neighboring peak one mass above.

• Low energy ions of the higher, abundant mass can produce low mass tailing of the peak shape, such that these ions contribute to the lower abundant mass of interest. The purpose of the retardation filter is to eliminate these low energy ions before they reach the detector, so reducing the tailing of the high abundance peak.

• The detector and the retardation filter are placed off-axis from the main instrument axis. The beam is deflected into the retardation filter and detector using a small but fast-switching dogleg deflector device.

Detector Calibration - AttoM

• The detector system of the AttoM comprises up to three different stages: Pulse counting electron multiplier, attenuated pulse-counting multiplier and Faraday (not available on our instrument)

• The instrument continuously monitors beam intensity and performs automatic switchover between detector modes as required. The crossover between the modes should be calibrated on a regular basis, depending on the applications and the required precision.

Attom High Resolution-ICP-MSDetector Assembly

Extended dynamic range – linearity is extremely important!

Sample Introduction System –Liquid

• Peristaltic pump or automated sampling system

• Nebulizer

• Spray chamber

• Plasma torch

Attom High Resolution-ICP-MSGlassware

Spray chamberNebulizer

Quartz torchTorch elbow

Attom High Resolution-ICP-MS

Spray chamber contained within Peltier cooling box, and torch elbow, next to the plasma hood.

Meinhardt nebulizer

Sample solution in line

End of quartz torch

Torch elbow

Attom High Resolution-ICP-MS

Peristaltic pump for introducing sample into spray chamber (when not in self-aspiration mode), and removing liquid sample waste (i.e., not nebulized).

Sample Introduction System

• Peristaltic Pump

– Pump liquid sample towards nebulizer and plasma torch

– Pump non-aspirated liquid into waste container

Sample Introduction System

• NEBULIZER

– Its function is to mix the liquid sample with the nebulizer gas (Argron) to produce a fine sample aerosol for introduction to the plasma discharge area.

Sample Introduction System• NEBULIZERS

– 3 main categories

• Pneumatic - concentric, cross flow, Babington, v-groove, Cone Spray

• Ultrasonic

• Direct insertion

Liquid sample – aspiratedeither in ‘free aspiration’mode or with a peristalticpump

Ar gas (0.6 to 1.2 L/min)

Aerosol production

-usually made of glass or variouskinds of polymers (for highly corrosiveliquids/samples)

Elemental Scientific Inc.MicroFlow PFA Nebulizer

• 100% Teflon • Self-aspiration:

– 20 µL/min– 50 µL/min– 100 µL/min– 400 µL/min

Concentric nebulizers

Concentric nebulizers

• “Self-actuating”

– Solutions are drawn up by the pressure drop generated as the nebulizer gas passes through the orifice – also referred to as “free-running” or “self-aspiration”. Thus, a peristaltic pump is not necessarily required.

• Advantage-– Generally, the ion signal is much more stable

Concentric nebulizers

• Disadvantages– cannot handle high total dissolved salts (TDS

- 0.25% m/v solids); i.e. 250 mg sample dissolved in 100 g of solution

– samples with different viscosities will have different flow rates

– liquid uptake tied to nebulizer gas flow– cannot easily increase flow for different

samples